1. Technical Field
This invention relates to medical treatment of stones arrested within a ureter, and more specifically, relates to catheter apparatus and methods for extracting ureteric stones without necessitating major surgery.
2. State of the Prior Art
For many years, individuals have been medically plagued by stones normally formed in renal calyces or the pelvis which pass to and are arrested within the ureter. Among other medical problems, these stones often cause blockage of urine produced by the kidneys with resultant ureteral colic and accompanying severe pain. It has long been recognized advantageous to treat patients having ureteric stones using procedures which do not require major open surgery involving incisions of the ureter. The dangers and complications associated with such surgery are well-known to any physician. Further, certain patients have medical histories prohibiting major surgery involving the ureteral area.
One early treatment procedure for ureteral stones was based on the concept that spasm in the smooth muscle of the ureteral wall prevented passage of the stone and passage of urine around the stone. Accordingly, the procedure involved use of spasmolytic drugs. However, it has been shown in various studies that spasm of the ureteral wall is not an overriding factor in the arrestment of ureteral stones. Therefore, spasmolytic drugs are not substantially effective.
Other early treatment procedures were based on the simple mechanical principle that a stone is arrested in the ureter when the force which is driving downward on the stone is smaller than the frictional force occurring between the stone and the ureteral wall. Historically, it was therefore thought that any treatment method which increased the downward driving force on the stone would be valuable to accomplish stone passage. Accordingly, patients were urged to increase fluid intake, thereby increasing the volume and resultant pressure of urine produced by the kidneys which was blocked above the stone. However, studies have shown that sudden pressure increase above the ureteral stone caused by discharge of urine from the kidneys actually causes an increased magnitude of frictional force between the stone and the ureteral wall. Therefore, passage of the ureteral stone is not facilitated by the increased urine, and further, severity of ureteral colic is actually increased.
The treatment procedures which merely attempt to increase the magnitude of downward forces above the arrested stone do not take into consideration the mechanical principles of frictional forces or the interrelationships of these principles to the physiological composition of the ureter. The frictional force at a point of contact between surfaces of the stone and the ureteral wall is equivalent to the product of a coefficient of friction and the force pressing between the stone and wall perpendicular to the plane of contact of the surfaces. A coefficient of friction for any two particular surfaces is dependent only on the materials involved and the "condition" of the surfaces.
As now understood in the physiological arts, the magnitude of the aforementioned perpendicular force between the stone and the ureteral wall is dependent upon several factors. These factors comprise: stone size, ureter width, and distensibility of the ureteral wall. Regarding stone size, the tensional force of the ureteral wall at the location of arrestment of the ureteral stone increases with respect to the diameter of the stone. As a general rule, stones having a diameter smaller than 2 millimeters pass through the ureter without causing substantial difficulty. Stones having a diameter greater than 2 millimeters but less than 4 millimeters also usually pass through the ureter but can cause associated ureteral colic. Various studies have shown that stones having a diameter of 4 to 6 millimeters independently pass through the ureter at a rate of approximately 50%. Stones having a diameter greater than 6 millimeters seldom pass through the ureter without medical treatment.
As known in the medical arts, the ureter comprises three sections of relatively narrow width. Frictional force between the stone and the ureter of course becomes greater at these narrow sections. Accordingly, it is at these sections that stones are often arrested. The relatively narrow sections are commonly known as the pyelo-ureteral conjunction, the passage over the crossing of iliac vessels and the uretro vesical junction which is the distal portion of the ureter.
Frictional forces between an object and a tubular structure surrounding the object increase inversely with the distensibility of the tubular structure. The distensibility of the ureteral walls is dependent on the individual distensibilities and arrangement of the materials composing the walls. The ureteral walls comprise approximately 70% fibrous collagen tissue having a distensibility substantially equivalent to 0.01% of the distensibility of smooth muscle fiber which substantially comprises the remainder of the ureteral wall composition. Accordingly, it is the collagen tissue that is determinative of the distensibility of the ureter and corresponding frictional force between a stone and the ureteral walls. Further, a moderate distension of the ureteral walls in the upper and middle portions of the ureter causes the collagen fibers to become contentrically arranged around the ureter lumen. Collagen fibers are always concentrically arranged in the lower portion of the lumen. This concentric arrangement offers substantial resistance to distension of the ureteral walls and, accordingly, produces large frictional forces in response to attempted distension.
With knowledge of the densities and arrangement of the collagen and muscle fibers in the ureter, it is surprising that relatively large stones will sometimes independently pass through the ureter. This phenomenon can be explained in view of the "visco-elastic" behavior of living tissue. In accordance with general mechanical principles, when a force is applied upon an elastic body, the deformation of the body is proportional to the magnitude of the force. This principle is commonly known as "Hooke's Law." The deformation of a visco-elastic material depends not only on the magnitude of the force applied to the material, but also upon the duration of the applied force. All living tissue, including collagen tissue, has visco-elastic properties. If a constant force is applied to such visco-elastic material, the material continuously "stretches" until it reaches an equilibrium state. This stretching and adjustment to equilibrium is commonly known as "creep." Further, if visco-elastic living tissue is stretched and then maintained at its new position for a substantial period of time, the tensional force generated by the living tissue will fall to a steady-state value. This particular principal is commonly known as "stress relaxation."
The aforementioned principles of creep and stress relaxation can readily be applied to arrestment of stones within a ureter. As a stone passes into the ureteral area, there is a distension of the ureteral walls which produces substantially large resistance and resultant frictional forces with the stone. Accordingly, the stone is arrested within the ureter. If the stone has been arrested at a particular position on the ureteral wall for a substantial period of time, the tensional force of the ureteral wall gradually ceases in accordance with the principle of stress relaxation. Accordingly, the frictional force between the stone and the ureteral wall will decrease and a stone can often pass from its arrested position towards the bladder.
However, other physiological phenomenon also occur when a ureteral wall is continuously deformed at a position of arrestment of a stone. As known in the medical art, a horizontal "bar" of the ureteral wall materials will often be formed below the stone, effectively blocking its passage. The size of this horizontal bar depends on the magnitude and duration of the driving force of the stone. It is this bar which often occurs when the pressure of urine above the stone increases.
Several types of devices exist in the prior art which are designed to extract ureteric stones without necessity of major open surgery. However, each of these devices suffers from limitations not found in the present invention and does not take into consideration all of the mechanical and physiological principles previously discussed herein. Further, substantially all of these devices are limited to extractions of ureteral stones in the lower one-third of the ureter with limited success to extract any stones above the lower third section. Additionally, these devices can produce varying degrees of trauma to the ureter and are therefor somewhat dangerous.
One such prior art device is commonly known as the Dormia ureteral stone dislodger. This device, and devices having limited modifications thereof, are utilized in a large percentage of medical centers now performing ureteral stone extraction. The basic Dormia stone dislodger comprises a catheter having a retractable spring wire mechanism. The spring wire mechanism is operable to activate a four wire pear-shaped basket having an opening therein to entrap a ureteral stone. The Dormia stone dislodger, like other prior art devices, is limited to extraction of ureteral stones in the lower third section of the ureter. Further, this dislodger is substantially limited in usefulness to extract stones having a diameter smaller than 8 millimeters.
Several other limitations of the Dormia stone dislodger also exist. The structure of the basket and the materials of which it is compressed can cause entrapment of the basket within the ureter and/or possible perforation of the ureteral wall. Additionally, the lack of expansive power of the wires of the pear-shaped basket to fully dilate the ureter has caused a substantially high failure rate when attempting stone extraction.
Another prior art device comprises a single catheter with two inflatable balloons located at concentric predetermined positions near the tip of the catheter. A syringe plunger attached to the lower end of the catheter provides variation in balloon size while the catheter is positioned in the ureter. The catheter is manipulated so that the arrested stone is between the balloons and is then slowly withdrawn with the lower balloon acting as a dilator and the upper balloon pushing the stone toward the bladder. With only a single catheter, this device does not allow the balloons to be separately manipulated. Further, the fixed positional relationship of the balloons and the concentric attachment of the upper balloon to the catheter limits the adaptability of the device to achieve extraction of relatively large stones, and, more particularly, the extraction of any size arrested above the lower third ureter section.